A monochromator having a diffraction grating is used in various devices, such as a spectrophotometer or a detector for a chromatograph (see Patent Literature 1). As one example of the commonly used spectrophotometers, a schematic configuration of a spectrofluorophotometer is shown in FIG. 1 (see Patent Literature 2). The spectrofluorophotometer 100 includes a light source unit 10, an excitation monochromator unit 20, a monitor unit 30, a sample chamber 40 and a fluorescence monochromator unit 50.
The light source unit 10 includes a light-condensing mirror 12 for condensing light emitted from a light source 11 (e.g. xenon lamp). The light condensed by the light-condensing mirror 12 travels through a first slit 13 into the excitation monochromator unit 20.
The light which has entered the excitation monochromator unit 20 is reflected by a reflection mirror 21 to a first diffraction grating 22 and dispersed in the wavelength direction by the grating 22. A portion of the light dispersed by the diffraction grating 22 passes through a second slit 23 and enters the monitor unit 30 as excitation light. The excitation monochromator unit 20 further includes a grating drive mechanism 24 for rotating the diffraction grating 22 about a rotation axis 22a. The wavelength of the excitation light can be arbitrarily set within a predetermined range of wavelengths by rotating the diffraction grating 22 through the grating drive mechanism 24.
In the monitor unit 30, a beam splitter 31 is placed on the path of the excitation light, whereby the excitation light is split into two directions. That is to say, a portion of the excitation light passes through the beam splitter 31 and reaches a sample cell 41 inside the sample chamber 40 after being condensed by a first lens 32. The other portion of the excitation light is reflected by the beam splitter 31 and condensed by a second lens 33, to be eventually detected by a control light detector 34 (e.g. a photodiode).
The excitation light which has reached the sample cell 41 causes the sample in the cell 41 to emit fluorescence. A portion of the fluorescence is condensed by a third lens 42 and enters the fluorescence monochromator unit 50.
A portion of the fluorescence which has entered the fluorescence monochromator unit 50 passes through the third slit 51 and falls onto a second diffraction grating 52, to be dispersed in the wavelength direction by the grating 52. A component of the dispersed light having a specific wavelength passes through a fourth slit 53, to be eventually detected by a fluorescence detector 54 (e.g. a photomultiplier). The fluorescence monochromator unit 50 further includes a grating drive mechanism 55 for rotating the diffraction grating 52 about a rotation axis 52a. The wavelength of the light to be detected by the fluorescence detector 54 can be arbitrarily set within a predetermined range of wavelengths by rotating the diffraction grating 52 through the grating drive mechanism 55.
To correctly perform an analysis of a sample using such a spectrofluorophotometer, both the wavelength of the excitation light passing through the second slit 23 and that of the detection light passing through the fourth slit 53 need to be set at the respective correct values. To this end, it is necessary to perform “calibration”, i.e. the task of correcting those wavelength values.
The wavelength calibration of a monochromator is performed using a standard light source, such as a sodium lamp or a mercury lamp, which generates a bright line spectral light to be used as the reference (see Patent Literatures 3 and 4). In the case of the previously described spectrofluorophotometer, the wavelength calibration is performed as follows: Initially, the light source unit 10 is configured so that a bright line spectral light to be used as the reference can be supplied to the monochromator. For example, this is achieved by replacing the light source 11 with a standard light source, or by switching the optical path to a built-in standard light source for wavelength calibration using a mirror or similar element (if the spectrofluorophotometer has such a light source). Subsequently, while the diffraction grating in the monochromator (the first or second diffraction grating 22 or 52) is being rotated, the output from the control light detector 34 or the fluorescence detector 54 is monitored so as to locate a rotational position of the diffraction grating (the first or second diffraction grating 22 or 52) at which the intensity of the diffracted light is maximized. Then, the rotational position giving the maximum intensity is related to the wavelength of the standard light source. Thus, the wavelength calibration is achieved.
In the previously described wavelength calibration method, while the diffraction grating is gradually rotated in steps of a preset wavelength resolution, the intensity of light received at a diffracted-light receiver (e.g. photodiode or photomultiplier) in the monochromator is detected at each rotational position, and the rotational position of the diffraction grating at which the intensity of the received light is maximized is located within a predetermined range of rotational positions. This approach has a problem in that, if the emission intensity of the standard light source for wavelength calibration changes, the intensity of the received light detected at each rotational position varies, which makes it impossible to correctly locate the rotational position at which the intensity of the received light is maximized, so that the wavelength calibration cannot be correctly performed.